The Inventions, Researches and Writings of Nikola Tesla

gradually under the weight of the moving parts until the end p, Fig. 283, strikes a stop on the top plate, B. The adjustment is such that when this takes place the rod R is yet gripped securely by the jaws e e. The further downward movement of the armature lever being prevented, the arc becomes longer as the carbons are consumed, and the compound magnet is weakened more and more until the clamping armature g releases the hold of the gripping-jaws e e upon the rod R, and the rod is allowed to drop a little, thus shortening the arc. The fine wire now receiving less current, the magnetism increases, and the rod is clamped again and slightly raised, if necessary. This clamping and releasing of the rod continues until the carbons are consumed. In practice the feed is so sensitive that for the greatest part of the time the movement of the rod cannot be detected without some actual measurement. During the normal operation of the lamp the armature lever L remains practically stationary, in the position shown in Fig. 283. Should it happen that, owing to an imperfection in it, the rod and the carbons drop too far, so as to make the arc too short, or even bring the carbons in contact, a very small amount of current passes through the fine wire, and the compound magnet becomes sufficiently strong to act as at the start in pulling the armature lever L down and separating the carbons to a greater distance. It occurs often in practical work that the rod sticks in the guides. In this case the are reaches a great length, until it finally breaks. Then the light goes out, and frequently the fine wire is[Pg 464] injured. To prevent such an accident Mr. Tesla provides this lamp with an automatic cut-out which operates as follows: When, upon a failure of the feed, the arc reaches a certain predetermined length, such an amount of current is diverted through the fine wire that the polarity of the compound magnet is reversed. The clamping armature g is now moved against the shunt magnet N until it strikes the releasing pole n'. As soon as the contact is established, the current passes from the positive binding post over the clamp r, armature g, insulated shunt magnet, and the helix p' upon the main magnet M to the negative binding post. In this case the current passes in the opposite direction and changes the polarity of the magnet M, at the same time maintaining by magnetic induction in the core of the shunt magnet the required magnetism without reversal of polarity, and the armature g remains against the shunt magnet pole n'. The lamp is thus cut out as long as the carbons are separated. The cut out may be used in this form without any further improvement; but Mr. Tesla arranges it so that if the rod drops and the carbons come in contact the arc is started again. For this purpose he proportions the resistance of part p' and the number of the convolutions of the wire upon the main magnet so that when the carbons come in contact a sufficient amount of current is diverted through the carbons and the part x' to destroy or neutralize the magnetism of the compound magnet. Then the armature g, having a slight tendency to approach to the clamping pole m', comes out of contact with the releasing pole n'. As soon as this happens, the current through the part p' is interrupted, and the whole current passes through the part x. The magnet M is now strongly magnetized, the armature g is attracted, and the rod clamped. At the same time the armature lever L is pulled down out of its normal position and the arc started. In this way the lamp cuts itself out automatically when the arc gets too long, and reinserts itself automatically in the circuit if the carbons drop together. [Pg 465]